The present invention relates to medical instrumentation apparatus, more particularly it relates to an improved delta modulation circuit for use in such medical instrumentation apparatus.
In the art of medical instrumentation, wherein sensing electrodes are attached to the patient to monitor certain body phenomena, it has been found necessary to provide a measure of isolation between the patient and the measuring, recording, and/or display apparatus, to prevent an inadvertent shock to or electrocution of the patient in the event of a malfunction of the measuring, recording, or display apparatus. In one form, that isolation circuitry has involved the use of a delta modulator to provide signal conversion.
In the conventional delta modulator, a variable voltage signal is compared with a quantized previous signal sample at a predetermined sampling rate. This produces a digitized output signal representative of the difference between the magnitude of the variable input signal and the previous sample. In such conventional delta modulators, a positive or a negative voltage reference signal is applied, depending upon the comparison of the previous signal with the present signal. The reference signal is applied through the sampling period to an integrator circuit to provide the quantized last signal sample. While such delta modulators have been used in systems in the past, the conventional delta modulator system as described includes a number of disadvantages which tend to limit the accuracy of such a system. One such deficiency is a voltage and current offset characteristic of the integrator. Such offset tends to produce an incorrect magnitude of the quantized reference signal. Second a separate reference voltage source is used for the positive and negative voltages. There is frequently a difference in the magnitude of the positive and negative reference voltage sources and this difference is also reflected in an incorrect value to the quantized reference signal. Because the signal is quantized by an integrator, the quantized signal is a time-dependent function. Accordingly, an additional deficiency is experienced in the presence of irregularities in the clocking signal. A further limitation which can result in an error is due to the finite time required to change the polarity of the quantized reference signal. Sine the quantizer is an integration circuit, a finite integrating time is of necessity a limiting factor on the sampling rate. When the input variable signal changes at a rate faster than the sampling rate, an inaccuracy in the resulting signal follows.